(Updates with visit by Brown, changes dateline)

By Adrian Croft

BASRA, Iraq, Dec 9 (Reuters) - British Prime Minister Gordon Brown paid an unannounced visit on Sunday to the troops whose mission he is winding down in Shi'ite southern Iraq.

South of Baghdad, a roadside bomb killed the police chief of Babel province, showing Shi'ite areas remain violent despite an overall reduction in bloodshed across Iraq.

Brown visited the last major outpost of British troops at an air base outside the city of Basra and said Britain would hand over responsibility within weeks for security in Basra province, the last of four it once patrolled in southern Iraq.

Britain now has just 4,500 troops in Iraq -- a tenth of the force that Brown's predecessor Tony Blair dispatched in 2003 to help the United States topple Saddam Hussein.

The British force is expected to be cut to 2,500 by mid-2008, leaving just a token foreign presence in the vast Shi'ite south of Iraq, even as Washington has sent an extra 30,000 troops this year to northern and central areas.

"The reason why security is so much better here, the reason why things have improved, is because of you, because of what you've achieved," Brown told troops assembled on the base.

Since withdrawing to the air base in September, British forces have faced few attacks.

But in a sign of how precarious the dwindling British presence is, journalists travelling with Brown were not permitted to report his visit until he was safely out of Iraq.

Brown, who spoke to Prime Minister Nuri al-Maliki by telephone, said the Iraqi leader recommended holding the security handover within two weeks. Maliki's office issued a statement saying that's Iraq's armed forces were ready.

The roadside bomb killed Major-General Qais al-Mamouri, police chief of mainly Shi'ite Babel province, just hours after U.S. military commanders had publicly praised his role.

"He is committed to securing Iraq for the people, the population. He does not see anything through a sectarian lens, it's all about Iraqi law, and the people see that," Colonel Tom James, commander of a U.S. combat brigade in north Babel, told a briefing in Baghdad before the assassination.

NUMEROUS ASSASSINATION ATTEMPTS

Police said Mamouri had survived six attempts on his life since becoming Babel police chief several years ago. Police immediately declared a curfew in the provincial capital Hilla.

Oslo-based historian Reidar Visser, a specialist in recent Iraqi history, said Mamouri had been in conflict with various Shi'ite militias in the past.

"For several years Mamouri stood out as an honest figure of authority in the mixed governorate of Babel, and had fought hard against militias regardless of their sectarian affiliations," Visser said in an email.

Other provincial police chiefs across Iraq have survived numerous assassination attempts. A roadside bomb killed the police chief of Diwaniya province in southern Iraq in August.

During the media briefing, U.S. commanders said about 1,400 U.S. troops would launch a fresh assault next week against Sunni Arab al Qaeda gunmen who are regrouping around Babel.

The offensive would target al Qaeda militants in small hamlets and fishing villages along the Euphrates River valley.

Babel is expected to be one of the next provinces to revert to the control of Iraqi security forces after Basra.

Washington says its build-up of forces this year has been successful in reducing violence, allowing the U.S. military to draw down own forces by more than 20,000 by mid-2008.

U.S. military spokesman Rear Admiral Greg Smith told a news conference in Baghdad that attacks had fallen 60 percent since June, when the American troop build-up was completed. (Additional reporting by Paul Tait, Dean Yates, Mussab al-Khairalla, Alaa Shahine, Peter Graff and Aws Qusay in Baghdad and Edmund Blair in Tehran; Editing by Myra MacDonald)

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